title: The first step of glycosylphosphatidylinositol biosynthesis is mediated by a complex of PIG-A, PIG-H, PIG-C and GPI1.
abstract: Biosynthesis of glycosylphosphatidylinositol (GPI) is initiated by transfer of N-acetylglucosamine (GlcNAc) from UDP-GlcNAc to phosphatidylinositol (PI). This chemically simple step is genetically complex because three genes are required in both mammals and yeast. Mammalian PIG-A and PIG-C are homologous to yeast GPI3 and GPI2, respectively; however, mammalian PIG-H is not homologous to yeast GPI1. Here, we report cloning of a human homolog of GPI1 (hGPI1) and demonstrate that four mammalian gene products form a protein complex in the endoplasmic reticulum membrane. PIG-L, which is involved in the second step in GPI synthesis, GlcNAc-PI de-N-acetylation, did not associate with the isolated complex. The protein complex had GPI-GlcNAc transferase (GPI-GnT) activity in vitro, but did not mediate the second reaction. Bovine PI was utilized approximately 100-fold more efficiently than soybean PI as a substrate, and lyso PI was a very inefficient substrate. These results suggest that GPI-GnT recognizes the fatty acyl chains of PI. The unusually complex organization of GPI-GnT may be relevant to selective usage of PI and/or regulation.
authors.affiliation: Department of Immunoregulation, Research Institute for Microbial Diseases, Osaka University, Japan.
authors.forename: R
authors.initials: R
authors.lastname: Watanabe
authors.forename: N
authors.initials: N
authors.lastname: Inoue
authors.forename: B
authors.initials: B
authors.lastname: Westfall
authors.forename: C H
authors.initials: CH
authors.lastname: Taron
authors.forename: P
authors.initials: P
authors.lastname: Orlean
authors.forename: J
authors.initials: J
authors.lastname: Takeda
authors.forename: T
authors.initials: T
authors.lastname: Kinoshita
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D002518:Ceramides / Q000494:pharmacology; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000302:isolation & purification; D004789:Enzyme Activation / Q000187:drug effects; D005982:Glutathione Transferase / Q000502:physiology; D017261:Glycosylphosphatidylinositols / Q000096:biosynthesis* / Q000378:metabolism; D006602:Hexosyltransferases; D006801:Humans; D046911:Macromolecular Substances; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D008969:Molecular Sequence Data; D010743:Phospholipids / Q000494:pharmacology; D029701:Saccharomyces cerevisiae Proteins; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity; D014162:Transfection
chemical_list: D002518:Ceramides; D018076:DNA, Complementary; C102317:GPI1 protein, S cerevisiae; D017261:Glycosylphosphatidylinositols; D046911:Macromolecular Substances; D008565:Membrane Proteins; C484451:PIGQ protein, human; D010743:Phospholipids; D029701:Saccharomyces cerevisiae Proteins; C080086:phosphatidylinositol glycan-class A protein; D006602:Hexosyltransferases; C101351:PIGC protein, human; D005982:Glutathione Transferase